The present invention relates to a circuit element utilizing spin resonance in a thin magnetic film such as a thin YIG (yttrium iron garnet) film formed on a non-magnetic substrate such as a GGG (gadolinium gallium garnet) substrate, and more particularly to the structure of a circuit element utilizing a magnetostatic wave, for suppressing a spurious mode and for enabling the circuit element to operate in a wide frequency range.
It has been proposed to use a thin-film ferrimagnetic resonator which is obtained in such a manner that a thin YIG film is deposited, by liquid phase epitaxy, on a non-magnetic substrate made of GGG and is then processed so as to have a desired shape, in a microwave oscillation circuit or the like. (Japanese patent application JP-A-2-13101 and others)
This thin-film ferrimagnetic resonator has advantages that the sharpness Q of resonance is high in a microwave frequency band and the resonance frequency of the resonator can be varied depending upon the intensity of a D.C. bias magnetic field applied perpendicularly to the thin ferrimagnetic film which is magnetically coupled with a transmission line.
Further, a circuit element utilizing a magnetostatic wave due to the spin resonance in a thin ferrimagnetic film has been proposed, in which a microwave transmission line is formed on the thin ferrimagnetic film through photolithographic techniques, to readily adjust the coupling between the thin ferrimagnetic film and the transmission line and to enhance the degree of coupling therebetween. (See Japanese patent Application JP-A-62-245704 which corresponds to U.S. Pat. No. 4,743,874, and others)
FIG. 2A shows an example of the abovementioned conventional circuit element utilizing a magnetostatic wave, and FIG. 2B shows a magnetostaticwave resonator used in the circuit element of FIG. 2A.
Referring to FIG. 2B, a magnetostatic-wave resonator 6 is formed in such a manner that a thin YIG film 3 is deposited on a GGG substrate 2 by liquid phase epitaxy, a gold or aluminum film is deposited on the YIG film 3, and one or more finger electrodes 5 and a pair of pad electrodes 4a and 4b are formed of the gold or aluminum film through photolithographic techniques so that the pad electrodes 4a and 4b are disposed on both sides of each finger electrode 5. The conventional circuit element utilizing a magnetostatic wave employs the resonator 6, and is constructed as shown in FIG. 2A. In FIG. 2A, reference numeral 1 designates the circuit element, 6 the magnetostatic-wave resonator, 7 a matching stub, 11 a conductor plate, 12a and 12b connecting plates, 13 an earthing plate, 14 a dielectric material, and 15 a micro strip line.
Owing to the mechanism of resonance, the above circuit element using the thin YIG film can operate at temperatures lower than the operation temperature of the well-known circuit element using a YIG sphere. Moreover, the former circuit element is relatively inexpensive, because the cumbersome step of forming the YIG sphere is not required.
When the band pass characteristics of the circuit element 1 of FIG. 2A are measured, a spurious mode frequently appears in the neighborhood of the lowest one of resonance modes, as shown in FIG. 3A. In many cases, the dependence of the resonance frequency of the spurious mode on an external magnetic field differs a little from the external magnetic field dependence of the resonance frequency of the lowest resonance mode. Accordingly, as shown in FIG. 3B, the positional relation between the spurious mode and the lowest resonance mode varies with resonance frequency. This fact will be explained below in more detail. When a resonance spectrum is observed while changing the resonance frequency of the lowest mode by varying the intensity of an external magnetic field gradually, a spurious mode appearing on one side of a peak due to the lowest resonance mode gradually approaches the peak, passes through the peak, and then moves to the other side of the peak. (See IEEE TRANS on MAGNETICS Vol. MAG-20, No. 5, Sep. 1984) Thus, in a case where the magnetostatic-wave resonator is used for forming a microwave oscillation circuit, when the operating point of the oscillation circuit passes through the resonance frequency of the spurious mode, there arises a problem that an oscillation frequency changes discontinuously as shown in FIG. 5A. Further, when the spurious mode and the lowest resonance mode overlap each other, there arises another problem that the shapness Q of resonance is greatly reduced.